In general, the term “organic light emitting phenomenon” refers to a phenomenon in which electric energy is converted to light energy by means of an organic material. The organic light emitting diode using the organic light emitting phenomenon has a structure usually comprising an anode, a cathode and an organic material layer interposed there between. Herein, the organic material layer may be mostly formed in a multilayer structure comprising layers of different materials, for example, the hole injecting layer, the hole transporting layer, the light emitting layer, the electron transporting layer, the electron injecting layer and the like, in order to improve efficiency and stability of the organic light emitting diode. In the organic light emitting diode having such a structure, when a voltage is applied between two electrodes, holes from the anode and electrons from a cathode are injected into the organic material layer, the holes and the electrons injected are combined together to form excitons. Further, when the excitons drop to a ground state, lights are emitted. Such the organic light emitting diode is known to have characteristics such as self-luminescence, high brightness, high efficiency, low drive voltage, wide viewing angle, high contrast and high-speed response.
The materials used for the organic material layer of the organic light emitting diode can be classified into a light emitting material and a charge-transporting material, for example, a hole injecting material, a hole transporting material, an electron transporting material and an electron injecting material, according to their functions. Further, the light emitting material can be divided into a high molecular weight type light emitting material or a low molecular weight type light emitting material according to the molecular weights, which ranges from a fluorescent material derived from the electron in the singlet excitation state to a phosphorescent material derived from the in electron in the triplet excitation state according to the light emitting mechanism. Further, the light emitting material can be divided into a blue, green or red light emitting material and a yellow or orange light emitting material required for giving more natural color, according to a light emitting color.
On the other hand, an efficiency of a device is lowered owing to maximum luminescence wavelength moved to a longer wavelength due to the interaction between the molecules, the deterioration of color purity and the reduction in light emitting efficiency when only one material is used for the light emitting material, and therefore a host/dopant system can be used as the light emitting material for the purpose of enhancing the color purity and the light emitting efficiency through energy transfer. It is based on the principle that if a small amount of a dopant having a smaller energy band gap than a host which forms a light emitting layer, excitons which are generated in the light emitting layer are transported to the dopant, thus emitting a light having a high efficiency. Here, since the wavelength of the host is moved according to the wavelength of the dopant, a light having a desired wavelength can be obtained according the kind of the dopant.
In order to allow the organic light emitting diode to fully exhibit the above-mentioned excellent characteristics, a material constituting the organic material layer in the device, for example, a hole injecting material, a hole transporting material, a light emitting material, an electron transporting material and an electron injecting material should be essentially composed of a stable and efficient material. However, the development of a stable and efficient organic material layer material for the organic light emitting diode has not yet been fully realized. Accordingly, the development of new materials is continuously desired.